1. Field of the Invention
This invention relates to a metal paste for use in multilayer wiring of an IC substrate, forming of an electrically conductive transparent film, bonding of a metal with a ceramic, or the like; to a process for producing the metal paste and to a method of making a metallic thin film using the metal paste.
2. Description of the Prior Art
One well-known process for producing ultrafine (superfine) particles of a metal for use in a metal paste of the type described above is a process of evaporation in a gas, the process comprising evaporating the metal in a depressurized atmosphere of an inert gas and collecting the evaporated metal on a cooling portion in the form of ultrafine particles having a particle size of 1000 angstroms (0.1 micron) or less.
The prior art process for producing the metal paste is to withdraw ultrafine metal particles produced, for example, in the above-described process of evaporation in a gas, into the atmospheric air and mix them with an organic solvent. More specifically, referring to FIG. 7, a crucible 6 in an evaporating chamber 1 is heated to evaporate a metal 7 in the crucible 6. The evaporated metal is introduced into a collection chamber 2 and collected in the form of ultrafine metal particles 12 on a cooling plate or a filter 11 maintained at a lower temperature by a coolant 13. The ultrafine metal particles 12 are withdrawn into the atmospheric air and mixed with an organic solvent to provide the metal paste.
With the metal paste produced by the prior art process, however, the particles may be adhered to one another in the course of production of the ultrafine metal particles, thereby producing agglomeration and/or chaining (i.e., connection in the form of a chain) of the particles. In addition, the surfaces of the particles may be oxidized and/or contaminated, resulting in an inferior dispersibility when they are mixed with the organic solvent. Further, because the particles are not individually separated, the density of the ultrafine metal particles in the paste may be lower.
When the metal paste is used, for example, for multilayer wiring in an IC substrate, it is necessary to form a pattern which is fine and has a uniform electric conductivity. When the conventional metal paste is used, however, a fine pattern cannot be formed well and the electric conductivity may not be uniform, because of an inferior dispersibility due to the presence of chained particles. In addition, in sintering the wiring formed from the metal paste, a higher temperature is required because of the oxidized surfaces of the particles. Further, shrinkage and/or cracks may be produced after sintering because of a lower density of the particles in the paste. This also results in an inferior uniformity in the electric conductivity.
Metallic thin films are conventionally used for making conductor circuits such as wiring, electrodes or the like of print circuit boards, thermal heads, condensers or the like. As the methods of making such metallic thin films, the following three are known.
A first method comprises the steps of: preparing an ordinary metallic paste by mixing particles of a metal or of an alloy thereof, whose particle size is 1 micron or more, with an organic solvent, a binder and an additive; and coating the paste on a substrate and drying and sintering them, thereby making a metallic thin film. In this method, the thickness of the metallic thin film is 5 microns or more.
A second method comprises the steps of coating a substrate with a metal-organic substance of liquid state in which a precious metal and an organic material are chemically combined; and drying and sintering them, thereby making a metallic thin film on the substrate. According to this method, it becomes possible to make a metallic thin film of 1 micron thick or less.
A third method comprises the step of making a metallic thin film by using a vacuum process such as a vacuum deposition method, a sputtering method or the like.
In making circuit patterns of a conductive body in the above-described methods, a screen printing method is employed in the first method, a screen printing or photoetching method is employed in the second method, and a photoetching method is employed in the third method, respectively.
In the above-described first method, while the content of metal particles in the metal paste is 50% or more, their particle sizes are 1 micron or more. Therefore, the metallic thin film has a film thickness of as much as 5 to 10 microns. This method thus has problems in that it cannot cope with promoting a decrease in the thickness of the film and that the amount of consumption of gold, platinum, palladium or the like, which is high in material cost, increases, resulting in a product which is high in an overall cost.
In the above-described second method, since the precious metal is present in the organometallic substance in the atomic condition, it is easy to promote a decrease in the thickness of the metallic film. However, since the content of the metallic composition is 20% at most, it is necessary to repeat 2 to 3 times the steps of coating, drying and sintering in order to obtain a thickness of 0.2 to 0.3 micron or more, resulting in a problem in that much time is required for making the film. Further, in this method, the organometallic substance to be used often contains harmful substances such as cyanogen and, therefore, there are problems in safety and hygiene in the step of sintering.
Furthermore, in the third method, since a vacuum batch treatment is required, the productivity is low. In addition, since an easy patterning method such as screen printing or the like cannot be employed, there is a problem in that the productivity is low also in this respect.
Considering the above description of the prior art, it is an object of this invention to provide a metal paste in which the problems associated with the conventional metal paste can be overcome, in which ultrafine metal particles are uniformly dispersed without agglomeration and chaining, and in which the surfaces of the particles are free from oxidation and contamination, and to provide a process for producing such a metal paste.
It is a further object of this invention, considering the problems associated with the above-described three methods of making metallic thin films, to provide a method of making a thinner metallic thin film in an easy way.